Method for producing silicon

ABSTRACT

A silicon production process which improves the production efficiency of trichlorosilane while an industrially advantageous output is ensured and the amount of the by-produced tetrachlorosilane is suppressed. This process does not require a bulky reduction apparatus for the by-produced tetrachlorosilane, can construct a closed system, which is a self-supporting silicon production process, can easily control the amount of the by-produced tetrachlorosilane and therefore can adjust the amount of tetrachlorosilane to be supplied to a tetrachlorosilane treating system when the tetrachlorosilane treating system is used.  
     This process comprises a silicon deposition step for forming silicon by reacting trichlorosilane with hydrogen at a temperature of 1,300° C. or higher, a trichlorosilane forming step for forming trichlorosilane by contacting the exhausted gas in the above silicon deposition step to raw material silicon to react hydrogen chloride contained in the exhausted gas with silicon, and a trichlorosilane first recycling step for separating trichlorosilane from the exhausted gas in the trichlorosilane forming step and recycling it to the silicon deposition step.

FIELD OF THE INVENTION

[0001] The present invention relates to a novel silicon productionprocess. More specifically, it relates to a process of producing siliconthrough a reaction between trichlorosilane (may be abbreviated as TCShereinafter) and hydrogen, which is capable of carrying out thetreatment of tetrachlorosilane (may be abbreviated as STC hereinafter)formed by a silicon deposition reaction industrially extremelyadvantageously.

[0002] Description of the Prior Art

[0003] High-purity silicon obtained from TCS can be produced from areaction between TCS and hydrogen. As an industrial production process,there is known a so-called “Siemens process” in which the surface of asilicon rod is heated and TCS is supplied to the rod together withhydrogen to deposit silicon on the rod in order to obtain a grownpolycrystal silicon rod.

[0004] The above deposition reaction is generally carried out at atemperature of 900 to 1,250° C., substantially 900 to 1,150° C. todeposit silicon stably, and STC and hydrogen chloride are by-produced bythe deposition reaction.

[0005] As for the amounts of STC and hydrogen chloride formed by thesilicon deposition reaction at the above temperature range employed bythe above Siemens process, STC is formed in an overwhelming amount.

[0006] When 1 ton of high-purity silicon is to be produced by theSiemens process at the above temperature range, STC is formed in anamount of 15 to 25 tons and hydrogen chloride is formed in an amount of0.1 to 1 tons.

[0007] STC by-produced by the silicon deposition reaction is a compoundwhich is chemically much more stable than TCS. As shown in the followingreaction formulas, as STC is contained in TCS more, the rate of thesilicon deposition reaction becomes lower, thereby greatly reducing theefficiency of the silicon deposition reaction due to an equilibriumimpediment factor as well.

[0008] Delay of Deposition Reaction Rate

TCS+H₂→Si+HCl+DCS+TCS+STC  (1)

STC+H₂→Si+HCl+DCS+TCS+STC  (2)

[0009] (DSC is dichlorosilane)

[0010] The yield of Si under the same reaction conditions is the formula(1):formula (2)=5:1.

[0011] Equilibrium Impediment

TCS+H₂→Si+HCl+DCS+TCS+STC

[0012] In the above formula, when STC in a formed system is existent ina raw material system, equilibrium tends to be left-sided. (law of LeChatelier)

[0013] Therefore, in a system for carrying out the silicon depositionreaction on an industrial scale, part or all of STC formed in largequantities by the silicon deposition reaction must be discharged to atreating system (to be referred to as “STC treating system” hereinafter)nearby or at a distance.

[0014] Examples of the STC treating system include a system forproducing fumed silica or quartz by hydrolyzing STC with oxyhydrogenflames and an epitaxial system for silicon wafers.

[0015] However, the consumption of STC in the STC treating system isaffected by demand for fumed silica or the like produced therefrom. Whenthe demand decreases, surplus STC which cannot be treated must beabandoned. Thus it is difficult to balance between the production ofsilicon and demand for STC, and a basic solution to the treatment of STCwhich is formed in large quantities has not yet been found.

[0016] To cope with the above problem, closed systems have been proposedby JP-A 52-133022 and JP-A 10-287413 (the term “JP-A” as used hereinmeans an “unexamined published Japanese patent application”) as aself-supporting process for reducing the output of STC and dischargingno STC. However, these systems are only based on an ideal system. Thatis, the system of JP-A 52-133022 provides a closed system technology forspecifying the composition of a gas used for the deposition of siliconto deposit silicon at a temperature of 900 to 1,250° C. in order tosuppress the by-production of STC. However, as seen in its Examples, asilicon deposition reaction system is placed under conditions close toan equilibrium state (ideal system) by making the amount of the suppliedgas extremely small for the reaction area. It is difficult to ensure anindustrially effective output under the above conditions. When thesupply of the raw material gas having the above composition is increasedto ensure the output, the reaction rate of TCS greatly lowers, whichmakes it difficult to carry out the silicon deposition reaction on anindustrial scale.

[0017] In order to produce silicon in an industrially advantageousamount at the above deposition temperature, the proportion of gaseoushydrogen must be reduced to improve the reaction rate of TCS, therebysharply increasing the by-production of STC.

[0018] Therefore, to carry out the deposition of silicon at a relativelylow temperature of 1,250° C. or lower on an industrial scale, theby-production of a large amount of STC is inevitable as described aboveand a technology for carrying out a closed system industrially has notyet been completed.

[0019] As a process for producing TCS from STC, JP-A 57-156318 proposesa process for obtaining TCS by converting STC into TCS through hydrogenreduction and then reacting the hydrogen chloride of the reaction gaswith low-purity silicon of a metallurgical grade (metallurgical-gradesilicon). However, even this process does not provide a solution to theproblem encountered in the step in which STC is formed in anoverwhelming amount.

[0020] Meanwhile, a process for carrying out a silicon depositionreaction at around 1,410° C. which is the melting point of silicon isproposed by JP-A 11-314996. However, researches are not made into thegas composition at the above deposition temperature as well as anindustrial process.

OBJECTS OF THE INVENTION

[0021] It is therefore a first object of the present invention toprovide a silicon production process which improves the productionefficiency of TCS while an industrially advantageous output is ensuredand the amount of by-produced STC is reduced.

[0022] It is a second object of the present invention to provide aself-supporting silicon production process which does not require abulky reduction apparatus for by-produced STC and enables theconstruction of a closed system.

[0023] It is a third object of the present invention to provide asilicon production process which can easily control the amount of theby-produced STC and therefore can adjust the amount of STC to besupplied to an STC treating system to any value when the STC treatingsystem is installed.

[0024] Other objects and advantages of the present invention will becomeapparent from the following description.

SUMMARY OF THE INVENTION

[0025] The inventors of the present invention have conducted intensivestudies to attain the above objects and have found that an industrialprocess which can reduce the amount of the formed STC to an extremelysmall value unattainable by the Siemens process by carrying out asilicon deposition reaction between TCS and hydrogen at a specific hightemperature range which has not been used for industrial production canbe established. Thus, the present invention has been accomplished basedon this finding.

[0026] That is, the first object and advantage of the present inventionare attained by a silicon production process comprising a silicondeposition step for forming silicon by reacting trichlorosilane withhydrogen at a temperature of 1,300° C. or higher, a trichlorosilaneforming step for forming trichlorosilane by contacting the exhausted gasin the above silicon deposition step to raw material silicon to reacthydrogen chloride contained in the exhausted gas with silicon, and atrichlorosilane first recycling step for separating trichlorosilane fromthe exhausted gas in the trichlorosilane forming step and recycling itto the silicon deposition step.

[0027] They have found that a closed system for dischargingsubstantially no STC to the outside of the process by reducing STC withhydrogen can be constructed because the amount of the by-produced STC isextremely small.

[0028] That is, the above second object and advantage of the presentinvention are advantageously attained by a silicon production processcomprising a silicon deposition step for forming silicon by reactingtrichlorosilane with hydrogen in a hydrogen/trichlorosilane molar ratioof 10 or more at a temperature of 1,300° C. or higher, a trichlorosilaneforming step for forming trichlorosilane by contacting the exhausted gasin the above silicon deposition step to raw material silicon to reacthydrogen chloride contained in the exhausted gas with silicon, atrichlorosilane first recycling step for separating trichlorosilane fromthe exhausted gas in the trichlorosilane forming step and recycling itto the silicon deposition step, a tetrachlorosilane reducing step forreducing tetrachlorosilane contained in the residue after the separationof trichlorosilane in the trichlorosilane first recycling step withhydrogen to obtain trichlorosilane, and a trichlorosilane secondrecycling step for recycling the exhausted gas in the tetrachlorosilanereducing step to the above trichlorosilane forming step.

[0029] Further, the inventors of the present invention have found thatthe amount of the by-produced STC can be adjusted to an extremely widerange without affecting the quality of the obtained silicon by changingthe hydrogen/TCS molar ratio in the silicon deposition reaction at theabove high temperature range and the amount of STC to be supplied to anSTC treating system can be thereby easily controlled even when the STCtreating system is installed.

[0030] Therefore, the above third object and advantage of the presentinvention are attained by a silicon production process comprising asilicon deposition step for forming silicon by reacting trichlorosilanewith hydrogen at a temperature of 1,300° C. or higher, a trichlorosilaneforming step for forming trichlorosilane by contacting the exhausted gasin the above silicon deposition step to raw material silicon to reacthydrogen chloride contained in the exhausted gas with silicon, atrichlorosilane first recycling step for separating trichlorosilane fromthe exhausted gas in the trichlorosilane forming step and recycling itto the silicon deposition step, a tetrachlorosilane reducing step forreducing a part of tetrachlorosilane contained in the residue after theseparation of trichlorosilane in the trichlorosilane first recyclingstep with hydrogen to obtain trichlorosilane, a trichlorosilane secondrecycling step for recycling the exhausted gas in the tetrachlorosilanereducing step to the above trichlorosilane forming step, and atetrachlorosilane supply step for supplying the balance oftetrachlorosilane supplied to the tetrachlorosilane reducing step to atetrachlorosilane treating system, wherein the amount oftetrachlorosilane to be supplied to the above tetrachlorosilane treatingsystem is changed in the above tetrachlorosilane supply step by changingthe molar ratio of hydrogen to trichlorosilane to be supplied to thesilicon deposition step.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIGS. 1 and 2 show silicon deposition apparatuses suitably usedfor carrying out the process of the present invention.

[0032]FIG. 3 is a graph showing the tendencies of the amount of STC andthe amount of hydrogen chloride at each deposition temperature.

[0033]FIG. 4 is a process diagram showing a typical embodiment of thepresent invention.

[0034]FIG. 5 is a process diagram showing another typical embodiment ofthe present invention.

PREFERRED EMBODIMENTS OF THE INVENTION

[0035] In the present invention, the silicon deposition step is to reactTCS with hydrogen at a temperature of 1,300° C. or higher, preferably1,300 to 1,700° C., more preferably the melting point of silicon orhigher and 1,700° C. or lower.

[0036] In the above silicon deposition step, the method of carrying outthe deposition of silicon industrially continuously is not particularlylimited. The method preferably uses the apparatus shown in FIG. 1, whichis the apparatus basically comprises (1) a cylindrical vessel 1 havingan opening 2 which is a silicon output port at the lower end, (2) aheater 3 which can heat the inner wall of the above cylindrical vessel 1from its lower end to any height at a temperature equal to or higherthan the melting point of silicon, (3) a chlorosilane feed pipe 4 forsupplying a chlorosilane A, which is open to a space 5 surrounded by theinner wall heated at a temperature of 1,300° C. or higher of the abovecylindrical vessel 1 and faces downward, (4) a seal gas feed pipe 6 forsupplying a hydrogen gas as a seal gas B into a space formed by theinner wall of the cylindrical vessel 1 and the outer wall of thechlorosilane feed pipe 4, and (5) a cooling material 9 mounted below theabove cylindrical vessel 1 with a space therebetween.

[0037] In order to carry out the collection of an exhaust gas D from thecylindrical vessel 1 efficiently in the above apparatus, the cylindricalvessel 1 and the cooling material 9 are preferably covered with a closedvessel 7 provided with an output pipe 12 for the exhaust gas D.

[0038] A seal gas C such as nitrogen, hydrogen or argon is preferablysupplied by a seal gas feed pipe 11 into a space formed by the outerwall of the cylindrical vessel 1 and the inner wall of the closed vessel7.

[0039] In the above apparatus, the heater 3 for heating the cylindricalvessel 1 is preferably a high-frequency coil. The cylindrical vessel 1is preferably made from a material which can be heated with highfrequency waves and has durability at the melting point of silicon. Ingeneral, carbon is preferably used. Carbon coated with silicon carbide,thermally decomposed carbon or boron nitride is preferred because it canimprove the durability of the cylindrical vessel and the purity of asilicon product.

[0040] In the above apparatus, a chlorosilane supplied from thechlorosilane feed pipe 4 may be mixed with hydrogen. A chlorosilane gasor a mixed gas of a chlorosilane and hydrogen is supplied into the space5 of the cylindrical vessel 1 together with hydrogen as a seal gassupplied by the seal gas feed pipe 6 and heated by the heater 3 todeposit silicon on the inner wall of the cylindrical vessel 1.

[0041] When the cylindrical vessel 1 is heated at a temperature of themelting point of silicon or higher, the deposited silicon flows downover the inner wall of the cylindrical vessel as a silicon molten liquidand is dropped from the opening 2 as a droplet 14 spontaneously.Therefore, the inside of the cylindrical vessel can be always kept in afixed state without carrying out the periodical operation of increasingthe temperature.

[0042] When the cylindrical vessel 1 is heated at a temperature of1,300° C. or higher and lower than the melting point of silicon, siliconseparates out as a solid. In this case, when the amount of the depositedsilicon reaches a certain value, the heat output is increased or thesupply of the gas is reduced to raise the temperature of the cylindricalvessel 1 to a temperature equal to or higher than the melting point ofsilicon in order to melt part or all of the deposit and drop it. Thussilicon can be collected and deposition can be carried out continuously.

[0043] In this text, as a reaction between TCS and hydrogen occurs onthe deposition surface of the above cylindrical vessel, the reactiontemperature is the heating temperature of the cylindrical vessel.

[0044] When the cylindrical vessel 1 is heated at a temperature aroundthe melting point of silicon, silicon may separate out partly in a solidstate and partly in a molten state. When the amount of the depositedsilicon reaches a certain value, the temperature is raised to melt partor all of the solid and drop it, thereby collecting it as in the abovemethod.

[0045] A droplet of the silicon molten liquid or partly molten solidsilicon falling from the above cylindrical vessel is dropped on thecooling material 9 which is a receptacle to be solidified and collectedas silicon 8.

[0046] When silicon is to be dropped as a molten liquid, before fallingsilicon is received by the cooling material 9 or while silicon isfalling before it is received by the cooling material 9, the siliconmolten liquid may be made fine by a known method.

[0047] The solidified silicon deposit dropped on the cooling material 9can be taken out from the closed vessel 7 after the deposition reactionis stopped, preferably taken out while the deposition reaction iscontinued. To collect silicon while the reaction is continued, there isemployed a method in which the temperature of the cylindrical vessel 1is adjusted to 1,300° C. or higher and lower than the melting point toprevent the silicon molten liquid from falling and a valve installedbetween the deposition reactor and a collection vessel is closed to opena collection unit or a method in which a grinder installed in acollection unit is used to apply mechanical force to the silicon depositsolidified in the collection unit to grind it to a certain extent andsilicon E is taken out from a silicon output port 13 located below thecooling material 9 intermittently.

[0048] In the reactor shown in FIG. 1, the raw material gas is suppliedinto the inside of the cylindrical vessel. As shown in FIG. 2, a reactorin which the cylindrical vessel 1 has a multi-structure having anopening at the bottom and the raw material gas A is supplied into aspace 15 formed between cylinders from above may be preferably used.

[0049] When the apparatus shown in FIG. 2 is used, in order to preventthe blockage of the above space by the deposition of silicon, it isrecommended to set the reaction temperature to the melting point ofsilicon or higher.

[0050] Heating means 16 such as a high-frequency generating coil or anelectric heater is placed in the central space of the multi-cylindricalvessel to heat an inner cylindrical vessel in particular to the full. Inthis case, an inert gas is preferably existent in the closed space forinstalling the heating means 16. The closed space may be evacuated. Aheat insulator (unshown) for protecting the heating means 16 may befurther provided.

[0051] In the process of the present invention, the reaction temperaturemust be 1,300° C. or higher. This is because the amount of STC formed inthe silicon deposition step is effectively reduced and the amount ofhydrogen chloride for facilitating the formation of TCS from STC isincreased.

[0052]FIG. 3 shows the trends of the amount of the by-produced STC andthe amount of the by-produced hydrogen chloride at 1,050° C., 1,150° C.,1,350° C. and 1,410° C. in the reaction when the molar ratio of hydrogento TCS is 10. As understood from FIG. 3, when the temperature is higherthan about 1,300° C., the amount of the by-produced STC greatlydecreases and the amount of the by-produced hydrogen chloride increases.

[0053] Although the reason why the reaction result obtained when thedeposition reaction temperature is 1,300° C. or higher and the reactionresult obtained when the conventionally proposed deposition reactiontemperature is 1,250° C. or lower differ from each other as describedabove is not elucidated yet, it is assumed that the temperature of theboundary film near the deposition surface is closely connected withthis.

[0054] That is, the supplied TCS is fully activated in thehigh-temperature boundary film to increase its conversion into siliconwhereas the activation of TCS is rather insufficient in thelow-temperature boundary film with the result that a reaction fordisproportionating two molecules of TCS into dichlorosilane and STCreadily occurs and a further reaction does not take place. In fact, inthe deposition reaction which is carried out at the temperature of thepresent invention, the amount of the formed dichlorosilane is muchsmaller than in the Siemens process of the prior art.

[0055] It has been found through studies on the present invention thatthe deposition reaction temperature which can attain the object of thepresent invention fully is 1,300° C. or higher. By carrying out thesilicon deposition reaction at the above temperature, the amount of theformed STC can be reduced to ½ or less that of the Siemens process ofthe prior art in some cases and to ⅓ in others. At the same time, theamount of the formed hydrogen chloride can be increased to 5 times ormore that of the Siemens process in some cases and 10 times in others.Moreover, the silicon deposition rate can be increased to 5 times ormore that of the Siemens process in some cases and to 10 times inothers, and the reaction rate of the raw material TCS can be increasedto 1.5 times or more that of the Siemens process in some cases and to 2times in others, thereby making it possible to produce a large amount ofsilicon with a very small-sized deposition reactor.

[0056] It is preferred to adjust the molar ratio (H₂/TCS) of hydrogen toTCS to be used in the above reaction to 10 or more, preferably 15 to 30so as to effectively reduce the amount of the formed STC and greatlyincrease the amount of the formed hydrogen chloride in the silicondeposition step.

[0057] Further, the pressure of the above reaction is not particularlylimited but preferably normal pressure or higher.

[0058] In the present invention, the TCS forming step is the step offorming TCS by contacting the exhausted gas in the silicon depositionstep to raw material silicon to react hydrogen chloride contained in thegas with silicon.

[0059] The exhausted gas in the silicon deposition step containshydrogen chloride and STC as the main products and small amounts ofdichlorosilane (to be abbreviated as DCS hereinafter) and oligomers ofchlorosilanes. The gas also contains unreacted TCS. When this mixed gasis contacted to raw material silicon, hydrogen chloride selectivelyreacts to form TCS. This reaction is an exothermic reaction which canproduce TCS energetically much more advantageously than an endothermicreaction for producing TCS by reducing STC with hydrogen.

[0060] As the above raw material silicon may be used knownmetallurgical-grade silicon which is generally used as a raw materialfor producing silicon without restriction.

[0061] Any reactor capable of contacting the raw material silicon to theexhausted gas from the deposition reaction may be used as the reactorused for the above reaction. For example, a fluidized bed reactor forreacting a raw material silicon powder with a gas while it is fluidizedby the gas is preferred for industrial-scale production. As means ofadjusting the temperature of the above fluidized bed reactor may be usedany known method. For example, a heat exchanger is installed internal orexternal to the fluidized bed, or the temperature of a preheating gas isadjusted.

[0062] In the trichlorosilane forming step, the temperature for startinga reaction between silicon and hydrogen chloride is almost 250° C.Therefore, the reaction temperature must be 250° C. or higher. Toimprove the yield of TCS, it is preferably 400° C. or lower. In order tocontinue the reaction industrially stably, the reaction temperature isparticularly preferably adjusted to 280 to 350° C.

[0063] In the present invention, the exhausted gas obtained from the TCSforming step is supplied to the TCS first recycling step to separate TCScontained in the gas and recycle it to the above silicon depositionstep. Hydrogen to be recycled is preferably obtained by removing partsof chlorosilanes from the gas. Various known methods may be used toseparate hydrogen from the chlorosilanes but this can be easily carriedout by cooling the gas industrially. To cool the gas, it may just passthrough a cooled heat exchanger or cooled with a condensed and cooledcondensate. These methods may be used alone or in combination. The abovecooling temperature is not limited if parts of chlorosilanes arecondensed but preferably 10° C. or lower, more preferably −10° C. orlower, the most preferably −30° C. or lower to improve the purity ofhydrogen. Most impurities contained in the raw material silicon, such asa heavy metal, phosphorus and boron, can be removed from hydrogen by theoperation of removing parts of chlorosilanes, and the purity of thedeposited silicon can be improved.

[0064] Parts of chlorosilanes are separated and the collected hydrogenhas sufficiently high purity but it may contain a relatively largeamount of a boron compound according to separation conditions.Therefore, according to the required purity of a silicon product, it isdesired that the boron compound should be removed from the hydrogen gas.The method of removing the boron compound is not particularly limitedbut a method in which a substance having a functional group such as —NR₂(R is an alkyl group having 1 to 10 carbon atoms), —SO₃H, —COOH or —OHis contacted to the above hydrogen gas is preferred. The simplest methodis to contact an ion exchange resin having any one of the abovefunctional groups to the hydrogen gas.

[0065] Any known method may be employed to separate TCS from the gasformed after the production of TCS. For instance, when the abovehydrogen is separated, TCS can be separated by purifying the condensedgas by distillation. The residue after the separation of TCS bydistillation purification includes a small amount of DCS as a light end,and STC, small amounts of chlorosilanes, oligomers of chlorosilanes andheavy metal compounds as a heavy end.

[0066] The above light end does not need to be separated from TCS.However, when it is separated, it is supplied to the STC reductionreaction together with STC or gasified to be supplied to the TCS formingstep again. Since the heavy end contains STC as the main component,after STC and heavy metal compounds are separated by a known method, STCis converted into TCS by the reduction step to be described hereinafteror treated in another treating step for its effective use.

[0067] To further remove the boron compound from the collectedchlorosilanes as a liquid according to the required purity of a siliconproduct, after the above solid or liquid compound having a functionalgroup is contacted to the chlorosilanes, the reaction product can bepurified by distillation as required.

[0068] The residue after the separation and collection of most of STCfrom the heavy end is generally neutralized and abandoned. In this case,as for chlorine lost by this, hydrogen chloride or a chlorosilane may besupplied into the system to compensate for the loss.

[0069] It is understood that the closed system in the present inventionincludes a mode that hydrogen which is inevitably reduced in quantity issupplied.

[0070] In the present invention, drive force for recycling is requiredto recycle the gas. Any known gas pressure device for generating driveforce may be employed. As for the installation position of the gaspressure device, it may be installed at an upstream of the TCS formingstep in which the apparatuses for the TCS forming step and the step ofseparating hydrogen from chlorosilanes can be reduced in size,preferably at an upstream of the silicon deposition step in which thetotal amount of substances for causing a trouble in the pressure deviceis the smallest.

[0071] The silicon production process of the present invention canreduce the amount of STC formed by the silicon deposition reaction to ¼that of the Siemens process of the prior art in some cases and to ⅕ inothers by a multiplication effect obtained by carrying out the silicondeposition reaction at 1,300° C. or higher and adjusting the molar ratioof hydrogen to TCS in the deposition reaction.

[0072] Therefore, the process for producing silicon with a closed systemfor converting the entire amount of STC formed in the silicon depositionstep into TCS to be recycled can be extremely advantageously realized.In addition, the size of the apparatus for recycling the gas can bereduced to about ½ or less that of the Siemens process owing to the highreaction rate of TCS and the high yield of silicon.

[0073]FIG. 4 is a process diagram showing the process for producingsilicon with the above closed system. As shown in the figure, theprocess comprises a silicon deposition step 101 for forming silicon byreacting TCS with hydrogen in a hydrogen/TCS molar ratio of 10 or moreat a temperature of 1,300° C. or higher, a TCS forming step 102 forforming TCS by contacting the exhausted gas in the above silicondeposition step to raw material silicon to react hydrogen chloridecontained in the gas with silicon, a TCS first recycling step 103 forseparating TCS from the exhausted gas in the TCS forming step andrecycling it to the silicon deposition step, an STC reducing step 104for obtaining TCS by reducing STC contained in the residue after theseparation of TCS in the TCS first recycling step with hydrogen, and aTCS second recycling step 105 for recycling the exhausted gas in the STCreducing step to the above TCS forming step.

[0074] A hydrogen/trichlorosilane separation step 201 for separatinghydrogen from chlorosilans by condensation is preferably carried outafter the above TCS forming step 102 as described above. In the TCSfirst recycling step 103, the separation of TCS is carried out by adistillation purifying step 202 for purifying a condensate solution fromthe above hydrogen/trichlorosilane separation step 201 by distillation.

[0075] In the above mode, the STC reducing step 104 is a step forconverting STC contained in the residue after the separation of TCS intoTCS by reacting it with hydrogen after the separation of hydrogen asrequired. As the reaction conditions may be used known conditionswithout restriction. To improve the conversion rate and amount of STCinto TCS, the reduction reaction temperature is adjusted to 1,300° C. orhigher, preferably 1,300 to 1,700° C., particularly preferably 1,410 to1,700° C. When the reduction reaction temperature is lower than 1,410°C., that is, lower than the melting point of silicon, the deposition ofsolid silicon in the inside of the reactor can be suppressed byadjusting the molar ratio of hydrogen to the supplied STC to 10 or less.When the reduction reaction temperature is 1,410° C. or higher, thedeposit is discharged to the outside of the system as a molten liquideven under conditions that silicon separates out, whereby the molarratio of hydrogen to STC can be adjusted without restriction.

[0076] The reactor used for this reaction is not limited to a particularstructure if it can attain a reaction temperature condition. Theapparatus shown in FIG. 1 or FIG. 2 used for the silicon depositionreaction is preferably used as a reduction reactor. That is, in thiscase, STC is supplied from the chlorosilane feed pipe 4.

[0077] In the present invention, the above TCS second recycling step 105is a step for recycling the exhausted gas in the STC reduction step 104to the above TCS forming step 102 to react hydrogen chloride containedin the gas with raw material silicon. The inventors of the presentinvention have found that the composition of an exhausted gas from thesilicon deposition reaction is very similar to the composition of anexhausted gas from the STC reduction reaction under conditions shown inthe present invention. That is, in the TCS forming step 102, theexhausted gases may be treated using a plurality of reactors or may betreated together by using a single reactor.

[0078] The silicon production process of the present invention has anadvantage obtained by carrying out the deposition of silicon at atemperature of 1,300° C. or higher and an advantage that the amount ofthe formed STC can be adjusted to a wide range from the above mentionedextremely small amount to the same amount as that of the Siemens processof the prior art by changing the molar ratio of hydrogen to TCS in thesilicon deposition step without changing the quality of silicon obtainedby the silicon deposition step.

[0079] That is, in the Siemens process of the prior art, thehydrogen/TCS molar ratio is controlled under fixed condition at a rangeof 5 to 10. It is known that when the molar ratio changes duringdeposition for some reason, the shape and the surface state of thedeposit deteriorate extremely, thereby reducing the value of a product,and that a sharp temperature distribution is partially formed duringdeposition with the result that the deposit is blown, thereby making itdifficult to continue deposition any more. Therefore, the operation ofgreatly changing the molar ratio is actually impossible industrially.

[0080] In contrast to this, since the deposition of silicon is carriedout at a high temperature close to a melting temperature of 1,300° C. inthe silicon production process of the present invention, a depositionreaction proceeds while silicon is partially molten or totally molten ata temperature of its melting point or higher. Silicon can be collectedby melting part or all of the deposit from a heated body.

[0081] Therefore, even when part of the deposition surface is molten bya change in the above molar ratio, it is substantially unnecessary totake into consideration the shape and surface state of the deposit andit is therefore possible to adjust the hydrogen/TCS molar ratio to anyvalue from any point of time.

[0082] That is, according to another embodiment of the presentinvention, as shown in the process diagram of FIG. 5, there is provideda silicon production process comprising a silicon deposition step 101for forming silicon by reacting trichlorosilane with hydrogen at atemperature of 1,300° C. or higher, a trichlorosilane forming step 102for forming trichlorosilane by contacting the exhausted gas in the abovesilicon deposition step to raw material silicon to react hydrogenchloride contained in the gas with silicon, a trichlorosilane firstrecycling step 103 for separating trichlorosilane from the exhausted gasin the trichlorosilane forming step and recycling it to the silicondeposition step, a tetrachlorosilane reducing step 104 for obtainingtrichlorosilane by reducing part of tetrachlorosilane contained in theresidue after the separation of trichlorosilane in the trichlorosilanefirst recycling step with hydrogen, a trichlorosilane second recyclingstep 105 for recycling the exhausted gas in the tetrachlorosilanereducing step to the above trichlorosilane forming step, and atetrachlorosilane supply step 106 for supplying the balance oftetrachlorosilane supplied to the tetrachlorosilane reducing step to atetrachlorosilane treating system, and the process in which the amountof tetrachlorosilane to be supplied to the tetrachlorosilane treatingsystem is changed by altering the molar ratio of hydrogen totrichlorosilane to be supplied to the silicon deposition step.

[0083] In the silicon production process of the present invention, whenthe molar ratio of hydrogen to TCS is made small, the amount of theformed STC can be made large and when the molar ratio is made large, theamount of the formed STC can be made small. Therefore, the deposition ofsilicon may be carried out by controlling the molar ratio according to arequired amount of STC in the STC treating system 203 in the STC supplystep. In general, the molar ratio of hydrogen to TCS can be controlledto a range of 5 to 30.

[0084] The above STC treating system includes all the apparatusescapable of making effective use of STC. Typical apparatuses include anapparatus for producing fumed silica by hydrolyzing the above STC withoxyhydrogen flames and an epitaxial apparatus for silicon wafers. Anyknown apparatus may be used as the STC treating system.

[0085] As understood from the above description, according to theprocess of the present invention, the amount of the formed STC can bereduced to an extremely small value which is totally impossible with theSiemens process by carrying out the silicon deposition reaction betweenTCS and hydrogen at a high temperature range of 1,300° C. or higher.Thereby, the process of the present invention makes it possible toconstruct a closed system that the formed STC is not discharged to theoutside of the process.

[0086] In the silicon deposition reaction at the above high temperaturerange, by changing the ratio of hydrogen to TCS, the amount of theformed STC can be adjusted to an extremely wide range without affectingthe quality of the obtained silicon. Thereby, even when a STC treatingsystem is used, the amount of STC to be supplied to the system can beeasily controlled and a well-balanced production mode can be employed.

EXAMPLES

[0087] The following examples are provided for the purpose of furtherillustrating the present invention but are in no way to be taken aslimiting.

Example 1

[0088] Silicon was produced in accordance with the process shown in FIG.4 as follows.

[0089] In the deposition step 101, the apparatus (deposition surfacearea of about 400 cm²) shown in FIG. 1 was used to supply TCS andhydrogen in a hydrogen/TCS molar ratio shown in Table 1 and heat theinner wall of the cylindrical vessel 1 at 1,450° C. so as to formsilicon. TCS was mixed with part of hydrogen and supplied from thechlorosilane feed pipe 4 and the remaining hydrogen gas was suppliedfrom the seal gas feed pipe 6 as a seal gas to ensure that the totalamount of hydrogen should be as shown in Table 1. A small amount ofhydrogen was supplied from the seal gas feed pipe 11 as a seal gas. Thereaction pressure was 50 kPaG.

[0090] Table 1 shows the amount of the deposited silicon, the amount ofthe formed STC and the amount of the formed hydrogen chloride in thesilicon deposition step. The amounts of the formed STC and hydrogenchloride were calculated by analyzing the exhausted gas from the silicondeposition reaction by gas chromatography.

[0091] The exhausted gas in the above silicon deposition step waspressurized at about 700 kPaG by a pressure device and heated to besupplied to the TCS forming step 102. In the TCS forming step, afluidized bed reactor for metallurgical-grade silicon as raw materialsilicon was used. The reaction conditions of the TCS forming stepincluded a temperature of 350° C., a pressure of 700 kPaG and a rawmaterial silicon charge of about 10 kg. The raw material silicon had anaverage particle diameter of about 200 μm and a purity of 98% andcontained metals such as iron, aluminum, titanium and calcium as themain impurities and also carbon, phosphorus and boron.

[0092] A dried hydrogen chloride gas was supplied to the TCS formingstep 102. The supply of the hydrogen chloride gas was used to maintainthe content of hydrogen in the system shown in Table 1. This amount wasbalanced with chlorine contained in the chlorosilanes to be dischargedto the outside of the system such as the heavy end extracted by adistillation purifying system and STC extracted as a surplus accordingto circumstances.

[0093] In the TCS forming step 102, hydrogen chloride formed in thesilicon deposition step 101 and the STC reducing step 104 to bedescribed hereinafter and hydrogen chloride supplied to maintain thecontent of chlorine in the system were reacted with the raw materialsilicon to form TCS as the main product and STC as a by-product.

[0094] After fine powders of the raw material silicon accompanied by thegas discharged from the TCS forming step 102 were removed by a filter,the gas was supplied to the hydrogen/chlorosilane separation step 201 tobe cooled to −30° C. in order to condensate parts of chlorosilanes,thereby separating a hydrogen gas. The separated hydrogen gas wassupplied to the above silicon deposition step 101 and the STC reducingstep 104 to be described hereinafter.

[0095] When the hydrogen gas from which parts of chlorosilanes had beenseparated was let pass through a vessel filled with 10 liters of an ionexchange resin having a substituent —N(CH₃)₂ and fully dried, the purityof the silicon deposit was 50 Ω·cm for P type. When the ion exchangeresin was not used, the purity of the silicon deposit was 1 Ω·cm for Ptype.

[0096] Meanwhile, the condensed chlorosilanes were supplied to thedistillation column of the TCS first recycling step 103 to separate intoTCS and the heavy end containing STC and heavy metals.

[0097] TCS and STC separated and purified in the TCS first recyclingstep 103 were gasified, TCS was supplied to the silicon deposition stepin an amount shown in Table 1, and STC was supplied to the STC reducingstep in an amount shown in Table 2. In the STC reducing step 104, areduction reaction was carried out by setting the hydrogen/STC molarratio to 10. As for the supply of hydrogen, the upper limit of the totalamount of hydrogen supplied to the silicon deposition step 101 and theSTC reducing step 104 was about 50 Nm³/H according to the limitation ofthe pressure device. A similar reactor to the reactor shown in FIG. 1was used as the reactor of the STC reduction reaction to supply a mixedgas of STC and hydrogen from the chlorosilane feed pipe 4. Otheroperation conditions were the same as the silicon deposition reaction.

[0098] Table 2 shows the reaction conditions of the STC reductionreaction and the amount of TCS formed by the reaction. The amount of theformed TCS was calculated by analyzing the exhausted gas from the STCreduction reaction by gas chromatography.

[0099] The gas exhausted from the STC reduction step was supplied fromthe TCS second recycling step 105 to the above TCS forming step 102.

[0100] Table 3 shows the amount of surplus STC, the amount of surplusSTC based on 1 kg of the produced silicon, the total supply of hydrogenfor the deposition reaction and STC reduction reaction and the supply ofhydrogen based on 1 kg of the produced silicon for evaluating the effectand economic efficiency of the above closed system.

[0101] The surplus STC refers to STC which must be discharged to theoutside of the system in the STC extraction step 106 as shown in FIG. 5because a larger amount of STC is still formed by the silicon depositionreaction or the like even when the maximum amount of STC which can besupplied to the STC reduction reaction is supplied.

[0102] As described above, according to the present invention, it isunderstood that the formation of STC can be suppressed in the silicondeposition step 101 and that a closed system for preventing theformation of surplus STC can be constructed even with a small-sized STCreduction reactor.

Examples 2 and 3

[0103] Silicon was produced in the same manner as in Example 1 exceptthat the molar ratio of hydrogen to TCS was changed as shown in Table 1in the silicon deposition step 101.

[0104] The amounts of the formed products in each step are shown inTables 1 to 3 in the same manner as in Example 1. It is understood thatan economical closed system can be constructed like Example 1.

Example 4

[0105] 15.6 kg/H of surplus STC could be formed as shown in Table 3 bychanging the molar ratio of hydrogen to TCS in Example 1 to 5 in thesilicon deposition step 101. This was calculated to be 14.9 kg ofsurplus STC based on 1 kg of the produced silicon. Thus, the same amountof STC as surplus STC obtained by the Siemens process to be describedhereinafter could be obtained.

Examples 5 and 6

[0106] The procedure of Example 1 was repeated except that thedeposition reaction temperature in the silicon deposition step 101 waschanged to 1,350° C., the molar ratio of hydrogen to TCS was changed asshown in Table 1 and the reduction reaction temperature of the STCreducing step 104 was changed to 1,350° C. as shown in Table 2. Thetemperature of the inner wall of the cylindrical vessel was raised to1,450° C. or higher for 5 minutes once every hour to continue thereaction while the deposit was molten and dropped intermittently.

[0107] As a result, a closed system could be constructed like Example 1as shown in Table 3.

[0108] As shown in the above Examples 1 to 6, according to the presentinvention, it is understood that an extremely wide range of operationfrom the construction of a perfect closed system to the acquisition of alarge amount of surplus STC is possible with apparatuses of the samescale by setting the deposition reaction temperature at 1,300° C. orhigher in the silicon deposition step 101 and changing the molar ratioof hydrogen to TCS. That is, according to the present invention, it ispossible to cope with a change in demand from an STC treating systemflexibly without changing the scale of the silicon production apparatus.

Comparative Example 1

[0109] A bell-jar type reactor (deposition surface area of about 1,200cm²) which is generally used in the Siemens process was used in thesilicon deposition step 101 and the STC reducing step 104 of Example 1.The deposition reaction temperature and the reduction reactiontemperature were set to 1,150° C. which was the upper limit temperatureable to be set industrially in the Siemens process of the prior art, andthe reaction pressure was 50 kPaG like Example 1.

[0110] The molar ratio of hydrogen to TCS in the silicon deposition step101 was 10 because the industrial upper limit for smoothening the shapeof the silicon deposit and maintaining a stable deposition reaction wasabout 10.

[0111] The other steps were carried out under the same conditions as inExample 1.

[0112] It is understood from the above Comparative Example that when thedeposition of silicon is carried out at a temperature equal to or lowerthan the melting point of silicon, particularly by the Siemens processof the prior art, a large amount of surplus STC is formed.

[0113] As understood from comparison between Comparative Example 1 andExample 4, the output of silicon in Example 4 is about 4 times largerthan in Comparative Example 1 even when the apparatuses of the samescale are used, and the process of the present invention has extremelyexcellent economic efficiency. TABLE 1 Amount of hydrogen Amount ofReaction for Supply of deposited Amount of Amount of Supply oftemperature reaction TCS Hydrogen/TCS silicon formed STC formed HCl HCl° C. Nm³/H kg/H molar ratio kg/H kg/H kg/H kg/H Ex. 1 1,450 30 12 150.51 4.6 1.3 0.30 Ex. 2 1,450 30 9.1 20 0.50 2.5 1.8 0.20 Ex. 3 1,450 306.0 30 0.37 1.5 1.3 0.14 Ex. 4 1,450 30 18 5 1.05 17 1.1 14.1 Ex. 51,350 23 14 10 0.50 5.5 1.3 0.37 Ex. 6 1,350 30 7.4 25 0.43 1.8 1.6 0.18C. Ex. 1 1,150 30 18 10 0.27 5.3 0.05 4.0

[0114] TABLE 2 Amount of hydrogen Reaction for Supply of Amount oftemperature reaction STC formed TCS ° C. Nm³/H kg/H kg/H Ex. 1 1,450 2015.8 4.8 Ex. 2 1,450 13 9.5 2.9 Ex. 3 1,450 8 5.9 1.8 Ex. 4 1,450 2015.8 4.8 Ex. 5 1,350 26 19.5 5.6 Ex. 6 1,350 10 7.5 2.2 C. Ex. 1 1,15020 15.8 2.8

[0115] TABLE 3 Amount of Total amount of hydrogen Total amount ofhydrogen surplus STC recycled for deposition recycled based Amount ofbased on 1 kg of reaction and STC on 1 kg of surplus STC producedsilicon reduction reaction produced silicon kg/H kg Nm³/H Nm³/H Ex. 1 00 51 100 Ex. 2 0 0 43 86 Ex. 3 0 0 38 103 Ex. 4 15.6 14.9 51 49 Ex. 5 00 48 96 Ex. 6 0 0 41 94 C. Ex. 1 4.0 14.9 51 189

1. A silicon production process comprising: a silicon deposition stepfor forming silicon by reacting trichlorosilane with hydrogen at atemperature of 1,300° C. or higher; a trichlorosilane forming step forforming trichlorosilane by contacting the exhausted gas in the silicondeposition step to raw material silicon to react hydrogen chloridecontained in said exhausted gas with silicon; and a trichlorosilanefirst recycling step for separating trichlorosilane from the exhaustedgas in the trichlorosilane forming step and recycling it to the silicondeposition step.
 2. The process of claim 1, wherein the molar ratio ofhydrogen to trichlorosilane is 10 or more in the silicon depositionstep.
 3. A silicon production process comprising: a silicon depositionstep for forming silicon by reacting trichlorosilane with hydrogen in ahydrogen/trichlorosilane molar ratio of 10 or more at a temperature of1,300° C. or higher; a trichlorosilane forming step for formingtrichlorosilane by contacting the exhausted gas in the silicondeposition step to raw material silicon to react hydrogen chloridecontained in said exhausted gas with silicon; a trichlorosilane firstrecycling step for separating trichlorosilane from the exhausted gas inthe trichlorosilane forming step and recycling it to the silicondeposition step; a tetrachlorosilane reducing step for obtainingtrichlorosilane by reducing tetrachlorosilane contained in the residueafter the separation of trichlorosilane in the trichlorosilane firstrecycling step with hydrogen; and a trichlorosilane second recyclingstep for recycling the exhausted gas in the tetrachlorosilane reducingstep to the trichlorosilane forming step.
 4. A silicon productionprocess comprising: a silicon deposition step for forming silicon byreacting trichlorosilane with hydrogen at a temperature of 1,300° C. orhigher; a trichlorosilane forming step for forming trichlorosilane bycontacting the exhausted gas in the silicon deposition step to rawmaterial silicon to react hydrogen chloride contained in said exhaustedgas with silicon; a trichlorosilane first recycling step for separatingtrichlorosilane from the exhausted gas in the trichlorosilane formingstep and recycling it to the silicon deposition step; atetrachlorosilane reducing step for obtaining trichlorosilane byreducing part of tetrachlorosilane contained in the residue after theseparation of trichlorosilane in the trichlorosilane first recyclingstep with hydrogen; and a trichlorosilane second recycling step forrecycling the exhausted gas in the tetrachlorosilane reducing step tothe trichlorosilane forming step; and a tetrachlorosilane supply stepfor supplying the balance of tetrachlorosilane supplied to thetetrachlorosilane reducing step to a tetrachlorosilane treating system,wherein the amount of tetrachlorosilane to be supplied to thetetrachlorosilane treating system is changed in the tetrachlorosilanesupply step by changing the molar ratio of hydrogen to trichlorosilaneto be supplied to the silicon deposition step.
 5. The process of claim 3or 4, wherein the reaction temperature in the tetrachlorosilane reducingstep is 1,300° C. or higher.